It is well known that under certain operational conditions and with certain fuels, gasoline powered internal combustion engines will exhibit a behavior called "knocking" or "pinging". These noises are audible oscillations generated by a shock wave in the compressed fuel-air mixture. It is an experimentally determined fact that the heat transfer to the piston and the cylinder walls of the engine is greatly increased during the occurrence of these oscillations and, as a result, these surfaces receive a detrimental thermal overload. Thus, any knocking in the engine is undesirable and should be prevented. It is desirable to be able to operate the engine within wide limits and as near as possible to the point of occurrence of engine knocking. Accordingly, it is useful to provide a sensor or detector which indicates the earliest occurrence of engine knocking reliably and early. Piezoelectric pressure indicators are known which are inserted into a special bore within the cylinder head of an internal combustion engine and which are in communication with the internal space of the cylinder. This manner of mounting a detector is difficult and expensive. Furthermore, the known sensors are broad-band detectors which respond to oscillations occurring over a wide range of frequencies. As a consequence, the desired signal which relates to engine knock and which occurs in a relatively narrow band of frequencies is hidden in a wide band of background noise. In order to use output signals of the known sensors, it is thus required to form the difference of the intensity at various times or to filter the generated signals. Both of these measures are relatively difficult and costly and the presence of additional circuitry tends to increase the sensitivity and failure probability of the entire apparatus.